In a conventional spray coating process for spray coating surfaces, such as the inner surface of cans, small particles of sprayed coating material adhere to the surface being sprayed and form a coating film. Some of the fine particles of coating material, however, do not adhere to the surface being coated and form a mist (called "overspray" herein) which floats in the vicinity of the spray apparatus. Some of the overspray sticks to the spray or coating gun and to its spray nozzle, and gradually accumulates to form a layer of coating material thereon. Sometimes, globs of the accumulated material fall into the spray and become a part of the coating on the surface being sprayed. These globs of coating material mar the appearance of the coated surface and can also cause a defect in the can when the globs do not cure in the allotted time.
Another problem caused by oversprayed coating material collecting on the spray nozzle is the partial blockage of the spray nozzle orifice and a resulting distortion in the spray pattern causing a portion of the sprayed surface to be left substantially uncoated.
To avoid these problems associated with oversprayed coating material, machine operators must periodically stop the coating process and clean the spray nozzle. In addition to having to periodically clean the nozzle, it is common for the operators to periodically coat the exposed surfaces of the spray gun, with the exception of the spray nozzle, with grease or animal fat to enable the gun to be more easily cleaned of overspray with a rag. Also, in some cases, cardboard spray shields have been placed on the spray gun body to prevent some of the overspray from accumulating on the spray gun. However, these spray shields are not only crude, but become soggy and quickly loose their effectiveness. Moreover, these cardboard shields do not protect the nozzle and the portion of the gun barrel or extension closest to the nozzle which is of particular importance.
In a prior art system for cleaning spray nozzles, as disclosed in Japanese Document No. 62-42688 assigned to Nordson Corporation, there is described a method and apparatus for cleaning spray nozzles wherein a hood is installed at the base of the spray coating nozzle. However, this hood is spaced from the spray nozzle and does not prevent overspray from accumulating on the spray nozzle itself or on the portion of the gun barrel or extension closest to the nozzle. This prior art document also shows a solvent spray nozzle positioned above or alongside the spray coating nozzle to spray a solvent on the spray coating nozzle immediately after a specified number of coating operations are completed to rinse off oversprayed coating material. However, when fewer than the specified number of coating operations are completed, typically at the end of a production run, the solvent is not sprayed onto the spray nozzle and the coating material can dry or skin over on the nozzle and clog it.
Another prior art spray gun system, having means for cleaning a coating spray nozzle, of a spray gun system is described hereinbelow.